EDM cuttable, high cBN content solid PCBN compact

ABSTRACT

The present disclosure relates to cubic boron nitride (cBN) cutting elements that have high cBN content and that are cuttable by electric discharge machining (EDM). A cutting element according to an embodiment includes a self-sintered polycrystalline cubic boron nitride (PCBN) compact, having a first phase of cubic boron nitride (cBN) particles and a ceramic binder phase with titanium compounds. The first phase occupies greater than 80% by volume of the self-sintered PCBN compact. The self-sintered PCBN compact has an electrical conductivity sufficient to be cuttable by electrical discharge machining.

FIELD OF THE INVENTION

The present disclosure relates to cubic boron nitride (cBN) cuttingelements that have high cBN content and that are cuttable by electricdischarge machining (EDM).

BACKGROUND

Sintered compacts made from cubic boron nitride (cBN) are used incutting tools and are known for their good wear resistance. To form sucha compact, the cBN particles are sintered at high pressure and hightemperature (HPHT sintering) to produce a polycrystalline cubic boronnitride (PCBN) structure. The cBN particles may be HPHT sintered in thepresence of a substrate material, which provides a metal catalyst thatinfiltrates into the cBN layer from the substrate and assists with theformation of PCBN and the intercrystalline bonding between the cBNgrains.

Alternatively, the cBN particles may be HPHT sintered without asubstrate present, in which case the resulting PCBN compact may bedescribed as “solid” or “self-sintering” or “self-sintered” or“free-standing.” A catalyst/binder material may be mixed with the cBNparticles prior to sintering in order to promote the formation of thePCBN structure during HPHT sintering, or the catalyst/binder materialmay be placed adjacent the cBN particles. Sintering without a substratecan be advantageous, as the substrate does not occupy valuable workingspace within the high pressure press, and the space can be fullyoccupied by the cBN mixture. As an example, a high pressure press mayhave a working volume of about 50 cm³. Also, the catalyst/bindermaterial may be uniformly mixed throughout the cBN mixture, rather thaninfiltrating into the cBN layer from a substrate, and as a result theself-sintered PCBN compacts may have more uniform compositions andmaterial properties.

However, the known catalyst/binder materials used for self-sinteringcreate PCBN compacts that are ceramic (dielectric) in nature and notconductive. As a result the self-sintered PCBN compact cannot be cut byelectric discharge machining (EDM). After sintering, it is oftennecessary to cut the sintered PCBN compact into a desired shape for aparticular cutting tool. Cutting by EDM is advantageous in manyapplications, as EDM cutting can reduce tool processing costs and allowfor more precise geometries to be produced. The laser cutting processmay produce a less uniform surface finish and less flat (i.e., planar)or perpendicular cut surfaces, resulting in additional finishing costs.The laser cutting process can also cause thermal damage. However EDMcutting requires that the material being cut is conductive orsemi-conductive.

The binder materials used to form self-sintered PCBN compacts havetypically not been conductive, and therefore the resulting PCBN compactcannot be cut by EDM. For example, one binder precursor material thathas been used to form self-sintering PCBN is Aluminum. After HPHTsintering the resulting self-sintering PCBN compact has an Aluminumceramic binder phase between the PCBN grains. This Aluminum ceramicbinder phase is non-conductive. PCBN compacts with other types of binderphases have been attempted in the past, but such compacts have beentypically limited to low cBN content, are not EDM-cuttable, and/or donot have sufficient hardness and strength properties for the intendedapplications.

Accordingly there is still a need for a high cBN content self-sinteringPCBN compact with a conductive or semi-conductive ceramic binder phase,that is EDM-cuttable, with desired material properties for an intendedapplication.

SUMMARY

The present disclosure relates to cubic boron nitride (cBN) cuttingelements that have high cBN content, are self-sintering, and arecuttable by electric discharge machining (EDM). In one embodiment, acutting element comprises a self-sintered polycrystalline cubic boronnitride (PCBN) compact, which comprises a first phase of cubic boronnitride particles and a ceramic binder phase comprising titaniumcompounds. The first phase occupies greater than 80% by volume of theself-sintered PCBN compact. The self-sintered PCBN compact has anelectrical conductivity sufficient to be cuttable by electricaldischarge machining.

A cutting element according to an embodiment includes a self-sinteredpolycrystalline cubic boron nitride (PCBN) compact, having a first phaseof cubic boron nitride (cBN) particles and a ceramic binder phase withtitanium compounds. The first phase occupies greater than 80% by volumeof the self-sintered PCBN compact. The self-sintered PCBN compact has anelectrical conductivity sufficient to be cuttable by electricaldischarge machining.

In another embodiment, a method of forming a self-sinteredpolycrystalline cubic boron nitride (PCBN) cutting element is provided.The method includes mixing a plurality of cBN particles with a binderprecursor to form a mixture. The mixture includes over 80% by volume cBNparticles. The method also includes HPHT sintering the mixture withoutsubstrate support, to form a self-sintered PCBN compact, and cutting theself-sintered PCBN compact by electrical discharge machining.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a representation of a mixture of cBN particles and a binderprecursor for HPHT sintering, according to an embodiment of the presentdisclosure.

FIG. 2 is a representation of a sintered polycrystalline structureaccording to an embodiment of the present disclosure.

FIG. 3 is a flowchart of a method of forming a self-sinteredEDM-cuttable PCBN compact according to an embodiment of the presentdisclosure.

FIG. 4 is a chart showing test results for several PCBN compactsincluding a compact according to an embodiment of the presentdisclosure.

FIG. 5 is a representation of a cutting tool insert tipped with piecescut from a PCBN compact, according to an embodiment of the presentdisclosure.

DETAILED DESCRIPTION

The present disclosure relates to self-sintering PCBN compacts that havehigh cBN content and that are cuttable by electric discharge machining(EDM). In one embodiment, a self-sintering PCBN compact includes highcBN content and a semi-conductive or conductive binder phase thatprovides good material properties for the sintered PCBN compact and alsoenables the compact to be cut by EDM. The sintered PCBN compact hassufficient electrical conductivity that it can be cut by EDM into acutting insert shape (such as cut by EDM into a cutting insert having aparticular thickness or a desired surface geometry).

An embodiment of the present disclosure is illustrated in FIGS. 1-2.FIG. 1 shows a powder mixture 10 including a cBN mixture 12 made up ofcBN particles of a desired size or range of sizes. The cBN mixture 12also includes a binder precursor 14 that is uniformly mixed with the cBNparticles. The binder precursor 14 includes the catalyst/binder materialthat facilitates the formation of PCBN during HPHT sintering. Asillustrated in FIG. 1, the cBN and precursor mixture 10 is placed into ahigh pressure press, and is sintered at high temperature and highpressure. For example, in one embodiment the powder mixture 10 ispressed at a pressure in the range of 3 to 6.5 GPa at an elevatedtemperature in the range of 1300-1500° C.

This HPHT sintering process creates a polycrystalline structure having anetwork of intercrystalline bonded cBN grains 16, with the precursormaterial forming a binder phase 18 remaining in the voids or gapsbetween the bonded PCBN grains 16, as shown for example in FIG. 2. AfterHPHT sintering, the sintered PCBN compact may be cut into a desiredshape for a particular cutting tool, such as by EDM cutting.

The EDM cutting process relies on some portion of the PCBN compact beingconductive or semi-conductive. EDM may also be referred to as wire EDM,spark machining, spark eroding, or wire erosion. EDM cutting works byremoving material by electrical discharges, or sparks. With high voltageapplied, a series of electrical current discharges are passed between anelectrode and the object being cut, causing a small amount of materialto be removed. This process works with materials that are electricallyconductive or at least semi-conductive.

In one embodiment, a self-sintering PCBN compact is provided that hashigh cBN content and a binder phase 18 that is conductive orsemi-conductive. The PCBN compact is formed by HPHT sintering a mixtureof cBN powder 12 and a binder precursor 14. The powder mixture includeshigh cBN content, such as about 80% cBN by volume, or above 80% cBN byvolume, or about 81% by volume, or above 81% by volume, or about 85% byvolume, or above 85% by volume, or between 80-95% by volume, or between85-95% by volume. In one embodiment the powder mixture includes about85% by volume cBN. In one embodiment the cBN particles have a size inthe range of about 12-22 microns, which is useful for forming PCBNcompacts for use as blanks for friction stir welding and high materialremoval rates in metal cutting. In another embodiment the cBN particleshave a size in the range of about 1-2 microns, which is useful forforming PCBN compacts for use in machining metal with fine surfacefinishing. In another embodiment, the cBN particles are in the range of2-4 microns in size, and in another embodiment they are sub-micron(i.e., in the range of 0-1 micron) in size. In another embodiment, thecBN particles are in the range of 3-6 microns in size, and in anotherembodiment 6-12 microns. Other cBN particle sizes and ranges of sizescan be used depending on the application. The cBN particle size rangesgiven herein are in accordance with the ANSI B74 20 standard. Forexample, a range of 2-4 microns means that D5 (5% of the particles) havea minimum particle size of 2 microns and D95 (95% of the particles) havea maximum particle size of 4 microns. As such, less than 5% of theparticles are less than 2 microns in size, and less than 5% of theparticles are greater than 4 microns in size.

The binder precursor 14 occupies the remaining volume percent of thepowder mixture, such as less than 20% by volume, or less than 15% byvolume, depending on the amount of cBN. The binder precursor 14 isselected such that the resulting binder phase 18 is conductive orsemi-conductive. In one embodiment, the binder precursor includesTitanium (Ti). In particular, in one embodiment the binder precursor istitanium aluminum carbide (Ti₃AlC). In one embodiment, the precursorcompound is ground to sub-micron sized powder particles (i.e., particleshaving average particle size of less than 1 micron) and is then blendedwith the cBN particles. In one embodiment, the oxygen level in thebinder precursor is kept low, such as less than 5% by weight (of thebinder weight). The low oxygen content promotes better binding betweenthe materials during HPHT sintering.

In one embodiment the cBN and binder precursor mixture includes 85% byvolume 2-4 micron cBN particles and 15% by volume sub-micron sizedparticles of Ti₃AlC. Other options for the binder precursor 14 includeTi₂AlC and Ti₂₋₃AlN. In one embodiment the binder precursor comprisesone or more of the following: Ti₃AlC, or Ti₃AlN, or Ti₂AlC, or Ti_(z)AlN(where z=2 or 3). Ti₃AlN may not be detectable, although present intrace amounts (typically below 1% by volume). The binder precursor maybe synthesized by a reaction of Ti, TiC, TiCN, and TiN with Aluminum. Inother embodiments, the cBN and binder precursor mixture may also include0-6% by volume Cobalt and/or 0-2% by volume WC. Cobalt may be added tofurther improve the cBN particle rearrangement and densification duringsintering, and to increase the conductivity of the PCBN compact. Thetungsten carbide (WC) may be added to increase the toughness of the PCBNcompact. However the addition of these materials is optional.

The mixture of cBN particles and the binder precursor particles is thenHPHT sintered without substrate support, to form a self-sintered PCBNcompact with a first phase of PCBN and a second binder phase between thePCBN grains (see FIG. 2). The self-sintered PCBN compact has high cBNcontent, such as above 80% cBN by volume, or above 81% by volume, orabove 85% by volume, or between 80-95% by volume, or between 85-95% byvolume. In one embodiment the self-sintered PCBN compact includes about85% by volume cBN, in the form of the first PCBN phase.

During HPHT sintering, the titanium-aluminum-carbide precursor reactswith the cBN and with oxygen within the powder mixture, and formsvarious compounds that form the binder phase 18. In one embodiment, thecomponents of the precursor react to form titanium carbide (TiC),titanium carbonitride (TiC_(x)N_(y)), titanium nitride (TiN), titaniumdiboride (TiB₂), aluminum nitride (AlN), and/or aluminum oxide (Al₂O₃).These compounds are formed during sintering, rather than provideddirectly as the binder precursor material. The HPHT sintering may bereferred to as reaction sintering, as the binder precursor breaks downand reacts with the cBN particles during HPHT sintering to form thebinder phase compounds. These compounds in the sintered binder phase 18(see FIG. 2) are formed by reaction during the HPHT sintering ratherthan being provided in the precursor mixture. Reaction sintering canpromote better hardness in the sintered material. For example, asintered compact formed with a binder precursor of TiCN and Al showed ahardness of about 3,000 kg/mm² (with 85% by volume cBN). A sinteredcompact formed with a binder precursor of Ti₃AlC according to anembodiment herein showed a hardness of greater than 3,200 kg/mm² (85% byvolume cBN).

In one embodiment, the binder precursor fully reacts with the cBN duringHPHT sintering, such that the precursor compound is no longer presentafter HPHT sintering. The components of the precursor compound fullyreact with the cBN particles to form other binder phase compounds, suchas those listed above. The sintered PCBN compact may be analyzed byx-ray diffraction, scanning electron microscope (SEM), or other knownmethods to identify the compounds that are present. In one embodiment,the pre-sintering binder precursor compound (such as Ti₃AlC) is nolonger present in the sintered PCBN compact, or is present in only traceamounts (typically less than 1% by volume).

The resulting binder phase 18 includes Titanium in the form of thereacted, sintered compounds (titanium carbide, titanium carbonitride,titanium nitride, titanium diboride). It is believed that when Aluminumis provided in the binder precursor (such as Ti₃AlC), the Aluminumreadily reacts with oxygen in the powder mixture, forming Al₂O₃, andthereby causing the Titanium to react with the cBN particles, formingTiN and TiB₂. In one embodiment the binder phase is predominantlytitanium nitride and titanium carbide. Titanium compounds aresemi-conductive, and thus the binder phase is electrically conductive.As a result, the self-sintered PCBN compact with this binder phase 18 isEDM-cuttable to form a desired cutting insert shape. The conductivity ofthe PCBN compact can be measured by its electrical resistance. In oneembodiment, the PCBN compact that is formed after HPHT sintering the cBNparticles with the titanium-aluminum-carbide binder precursor has anelectrical resistivity of approximately 0.5×10⁻² ohm-m (Ω-m). In oneembodiment, a self-sintered PCBN compact includes an electricalresistivity p of less than about 0.5×10⁻² ohm-m (Ω-m). In oneembodiment, a self-sintered PCBN compact includes an electricalresistance of less than about 10⁻² ohm-m (Ω-m). This low resistivityenables the PCBN compact to be cut by EDM. Electrical resistivity can bemeasured from an EDM-cut bar using a four-point method.

In one embodiment, the sintered binder phase 18 is devoid of elementalTitanium, or includes elemental Titanium only in trace amounts. TheTitanium from the binder precursor fully reacts with the cBN particlesto form titanium boride, titanium carbide, and titanium nitride in thebinder phase. These titanium compounds are stable and have good hardnessfor cutting tools. Additionally, they are semi-conductive. This binderphase gives the self-sintered PCBN compact good material properties forcutting tool applications, while also enabling the PCBN compact to beEDM-cuttable. In one embodiment the self-sintered PCBN compact has aVickers hardness of greater than 3,200 kg/mm². In one embodiment, theself-sintered PCBN compact has a 3-point average bending strength of1080 MPa (tested with a span of 8.3 mm, a width of 1.2 mm, and athickness of 1.0 mm).

In one embodiment, the binder precursor 14 reacts with the cBN to form abinder phase 18 that is ceramic in nature, rather than metallic, therebyproviding more thermal stability in the self-sintered PCBN compact,while also providing electrical conductivity. In some applications, ametallic binder phase is less thermally stable than a ceramic binderphase. A metallic binder phase can expand at high temperature, causingcracking in the PCBN layer. The metals are also more likely to bereactive with the workpiece material that is being machined by the PCBNcutting insert. Also, PCBN compacts with metallic binder phasestypically rely on a substrate during HPHT sintering to provide the metalinfiltrant, and thus these compacts are not self-sintering. TheTitanium-based binder phase 18 of embodiments here is conductive but isstill ceramic in nature, providing chemical and thermal stability.

The Titanium constituents also provide electrical conductivity, so thatthe binder phase of the self-sintered PCBN compact is within theconductive range required by EDM cutting, for example, having anelectrical resistivity below about 10⁻² ohm-m (Ω-m). The titaniumaluminum carbide precursor (Ti₃AlC) is believed to provide goodconductivity because the molar ratio of Titanium to Aluminum is 3.Therefore, there will be more free Ti than Al released and then reactedwith the cBN powder to form TiB₂ and TiN compounds. The Ti compounds aresemi-conductive, while the Al compounds are not conductive. Thus inembodiments herein, the binder phase 18 includes more Ti compounds thanAl compounds, in order to make the sintered PCBN cuttable by EDM. Thebinder phase is predominantly titanium nitride and titanium carbide,which form a conductive network through the PCBN structure, providingthe PCBN compact with sufficient electrical conductivity for EDMcutting. In one embodiment, the ratio of Ti to Al in the self-sinteredPCBN compact is 3, and in another embodiment it is 2.

In one embodiment, a method of forming a self-sintering EDM-cuttablePCBN compact is provided, as shown in FIG. 3. The method includes mixingcBN particles with a binder precursor to form a uniform cBN andprecursor mixture (112). The cBN particles occupy at least 80%, such asabout 85%, of the mixture by volume. The cBN particles and the binderprecursor are provided in powder form and are mixed together by asuitable mixing procedure such as ball mixing, or attritor milling.Optionally, the method includes subjecting the cBN and precursor mixtureto a vacuum treatment (114). In one embodiment this step includesplacing the powder mixture into a vacuum furnace, applying a vacuum, andheating the furnace. In one embodiment the furnace is heated to about1,000° C. for about one hour. The vacuum promotes initial reactionbetween the binder precursor and the cBN particles, thereby making thepowder mixture more stable for operation. The binder precursor compoundmay partially decompose or break down, and the free atoms can begin toreact with the cBN particles. This step may be described as apre-sintering reaction.

After subjecting the mixture to a vacuum, or after mixing the cBN andprecursor particles together if the vacuum step is omitted, the methodthen includes HPHT sintering the free-standing cBN and precursormixture, without a substrate, to form a self-sintered PCBN compact(116). The HPHT sintering creates PCBN compact with a binder phase ofcompounds that formed from reactions between the binder precursor andthe cBN. The method then includes cutting the PCBN compact into asuitable cutting insert by an EDM cutting method (118).

As an example, the EDM cutting step may include cutting theself-sintered PCBN compact into one or more slices that have a thicknessof about 4.8 mm, or about 3.2 mm, or about 1 mm, or in the range of 1 mmto 50 mm, or other thicknesses, depending on the application. Thesesliced wafers can then be brazed onto a carbide body to form a cuttinginsert. The cutting insert may be used in applications where high wearresistance is desired, such as machining cast iron (or grey iron) andsuper alloys (such as nickel-based super alloys). EDM cutting can beused to create a uniform and perpendicular peripheral cut on the PCBNinsert.

FIG. 4 shows a plot of testing results according to one embodiment ofthe present disclosure. Several PCBN cutting elements were compared bysubjecting them to a cast iron turning test. The testing machine was aMori Seiki SL-25 CNC lathe, and the workpiece material was Class 35 greycast iron, with 200 Brinell hardness (BHN). The turning speed was 3,500surface feet per minute (sfpm), and the feed rate was 0.020 inches perrevolution. The depth of cut was 0.015 inches. The work piece wallthickness was 1.32 inches. The test was performed in dry conditions.

The PCBN compacts that were subjected to the test are summarized inTable I below. The bending strength of each PCBN compact is alsoprovided for comparison.

TABLE I Bending Strength PCBN Compact Description (MPa) Comparative ACarbide-backed PCBN compact (not available) with metallic binder phaseComparative B Carbide-backed PCBN compact (not available) with metallicbinder phase Comparative C Carbide-backed PCBN compact 725 MPa withcermet binder phase Comparative D Carbide-backed PCBN compact 1,100 MPawith cermet binder phase New PCBN Grade E Self-sintered PCBN compact1,080 MPa with 85% by volume cBN, according to an embodiment of thepresent disclosure

The PCBN grade E insert was brazed with a self-sintered PCBN compactformed by HPHT sintering 85% by volume 2-4 micron cBN particles and 15%by volume Ti₃AlC particles. The grade E is a self-sintered high contentPCBN compact that is EDM-cuttable.

The number of passes versus the wear on each cutting element is plottedin FIG. 4. As shown in FIG. 4, the grade E out-performed the othercutting elements in the test, enduring the largest number of passes withthe least amount of wear.

FIG. 5 shows a cutting tool insert 120 tipped with pieces 110 cut from aPCBN compact, according to an embodiment of the present disclosure. Thecutting insert 120 includes a cemented carbide insert body 112, and thePCBN tip pieces 110 cut from the PCBN compact are brazed to the body 112at the corners of the body.

Relative sizes are exaggerated in the figures for clarity, and are notnecessarily to scale.

Although the present invention has been described and illustrated inrespect to exemplary embodiments, it is to be understood that it is notto be so limited, since changes and modifications may be made thereinwhich are within the full intended scope of this invention ashereinafter claimed.

What is claimed is:
 1. A method of forming a self-sinteredpolycrystalline cubic boron nitride (PCBN) cutting element comprising:mixing a plurality of cBN particles with a binder precursor to form amixture, the mixture comprising over 80% by volume cBN particles; HPHTsintering the mixture without substrate support, to form a self-sinteredPCBN compact; and cutting the self-sintered PCBN compact by electricaldischarge machining; in which the binder precursor comprises one or moreof the following: Ti₃AlC, Ti₂AlC, Ti₃AlN.
 2. The method according toclaim 1, further comprising heating the mixture under vacuum prior toHPHT sintering.
 3. The method according to claim 1, further comprisingpartially reacting the binder precursor with the cBN particles prior toHPHT sintering.
 4. The method according to claim 1, wherein the HPHTsintering comprises reaction sintering to form a binder phase in theself-sintered PCBN compact.
 5. The method according to claim 1, whereinthe mixture comprises over 85% by volume cBN particles.
 6. The methodaccording to claim 1, including mixing the cBN particles and the binderby means of attritor milling.
 7. The method according to claim 1,wherein the binder precursor is Ti₃AlC.
 8. The method according to claim1, wherein the binder precursor has a mean grain size of less than 1micron.
 9. The method according to claim 1, wherein the binder precursorcontains less than 5% by weight of oxygen based on the binder weight.